1. Field of the Invention
The invention relates to an electrostatic device, and more particularly to an electrostatic device for use in an electro-photographic appliance such as a photocopier.
2. Description of Related Arts
As shown in FIGS. 11 and 12, an electrostatic device using a scorotron has been employed in conventional electro-photographic devices such as a laser printer or a photocopier.
The scorotron electrostatic device 150 has a shield casing 152 having a U-shaped cross section with an open face. A discharge wire 156 is provided in the center of the shield casing 152 between insulation blocks 154a and 154b on either end of the shield casing 152. Grid electrodes 158 are provided on the open face of the shield casing 152. The grid electrodes 158 are grounded via a varistor 160 with a voltage rating of about -680 volts.
When the scorotron electrostatic device 150 as described is used, the open face (the face with the grid electrodes) of the shield casing has to be kept parallel with the photo-sensitized drum 162 and a direct current voltage of -6 kv must be applied to the discharge wire 156 under fixed current control. In this condition, corona discharge occurs around the discharge wire 156, and negative ions created in the corona discharge pass through the grid electrode 158 and reach a photo-sensitized drum 162 giving an electrostatic charge to the surface of the photo-sensitized drum 162. As described above, since the grid electrodes 158 are grounded via a varistor 160, the ions flow to ground rather than to the photo-sensitized drum 162 when the potential on the surface of the photo-sensitized drum 162 nears the voltage rating of about -680 V.
However, the described scorotron electrostatic device has various shortcomings. First, from an environmental point of view, the scorotron electrostatic device has a fault in that the device ionizes oxygen in the atmosphere and creates ozone. The electrostatic device used in a laser printer has to be electrified with a negative charge due to the characteristic of toner particles, and the amount of ozone created in the corona discharge is significantly greater (by one decimal place) when the device is electrified with a negative charge rather than with a positive charge. Moreover, the amount of ozone created in the device is dependent on the current flow through the wire. The scorotron electrostatic device needs a current flow of -400 to -500 .mu.A on the wire to collect a current flow of several-tens .mu.A for properly electrifying the photo-sensitized drum. As a result, it creates a good deal of ozone. The density of the ozone reaches as high as 10 ppm when measured near the electrostatic device. Consequently, conventional laser printers had an ozone filter set in the exhaust duct for removing the ozone.
The low efficiency in current use mentioned above has resulted in a power unit having a large capacity, an ozone filter and an exhaust fan, thereby substantially increasing the cost of the products.
A second shortcoming is that silicon oil, used for removing toner in the fixing unit, evaporates into the air and is oxidized to be silicon oxide (SiO.sub.2) that remains on the wire. The silicon oxide adhering to the wire causes an increase in the impedance on the surface of the photo-sensitized drum, interfering discharge, and resultant sag in the initial voltage at the surface of the drum that has a negative influence on the quality of the printed characters.
To solve the above-identified problems, a surface discharge device as shown in FIG. 13 is proposed. The surface discharge device 164 has an electrode 168 on a substrate 166 made of, for example, glass, and a resistance film 170 thereon. With this construction, applying voltage to the electrode 168 triggers the corona discharge over the surface of the resistance film 170, and the ions generated in the corona discharge electrify the photo-sensitized drum 162.
The surface discharge device 164 is manufactured, for example, by sputtering tantalum (Ta) to form a thin film on the surface of the glass and exposing to nitrogen (N) to form a tantalum nitride (TaN) resistance film on the surface of the electrode. Material other than TaN, such as titan oxide (TaO.sub.2), can be used as a substitute Besides the sputtering method, amorphous silicon with impurities doped in the chemical vapor deposition (CVD) method can be used.
The electrostatic device using the surface discharge device 164, as described, can make an efficient use of the current, produce a lesser amount of ozone and requires a smaller power supply unit.
The conventional surface discharge electrostatic device had another inherent problem, that is, it had difficulty in controlling the resistance of the resistance film 170. Because the resistance film 170 has the electrode 168 on the side opposite to the discharge surface, it is extremely difficult to keep the resistance value in an optimal range by controlling the volume resistance measured across the thickness of the resistance film 170.
If the resistance is too low, a streamer discharge instead of a corona discharge occurs, thereby failing to properly electrify the photo-sensitized drum 162. The resistance film 170 must be made thicker to obtain a proper resistance value. However, the optimal resistance has a narrow range and forming thicker film using the sputtering process is costly.
What is worse, a tiny flaw on the resistance film 170 results in the current flowing from the electrode to the tiny spot which produces a concentration of the electric field. Under this condition, the streamer discharge occurs and the electrostatic device fails to electrify the photo-sensitized drum 162 properly.